Electronic circuits are fabricated by various deposition processes, one of which is by the screen printing of conductive, resistive, and dielectric inks in specific patterns on a ceramic substrate. The conductive, resistive, and dielectric inks, known as thick film inks, are prepared from finely divided powders (about 0.1-20 microns) of conductive, resistive, or dielectric materials dispersed in an organic vehicle.
The circuits may be a single layer of conductive and resistive material, or may be multilayers of conductors separated by layers of dielectric, in which the conductors are connected through vias in the dielectric layers. Conductor and resistor patterns form the top layer. As the microelectronics industry increasingly demands smaller geometries and faster assembly times, it becomes advantageous to be able to screen print finer features onto a circuit board and to reduce the total number of layers that must be printed to form the circuits.
The organic vehicle for thick film inks should have good wetting properties to thoroughly disperse the powder and allow it to pass through the screen more readily. The vehicle should also have good thixotropic properties, meaning the ink exhibits a drop in viscosity during the shear of printing and a full recovery after removal of the shear. This prevents the ink from spreading once it is deposited on the circuit board.
In the case of dielectric inks, the number of layers can be reduced if the solids content of the ink is increased. When the solids content is increased, however, the vehicles in current use do not sufficiently wet out the powders, and the ink becomes tacky and does not transfer well from the screen during the printing operation. If sufficient surfactants are added to counteract the tackiness and poor transfer properties, the ink will then exhibit considerable shear thinning during the screen printing process without adequate recovery immediately after shear is removed. This results in the smudging or closing of the small vias in the dielectric layers of the circuits.
In the case of conductive inks where heavier (than dielectric) metallic powders are used, the vehicle must be able to fully disperse the powder and keep it from settling. If the powders are not fully dispersed, the printed lines will not be well defined or have a smooth surface. Good line definition allows for the printing of denser circuits, and surface smoothness promotes superior wire bonding and soldering of the conductors. The requirement for smooth lines is usually met by lowering the viscosity, which results in loss of definition. Achieving both requirements is difficult with vehicles now in use.
To these difficulties can be added the problem of solvent loss during long screen printing runs, which makes the ink tacky on the screen and adversely affects the quality of the circuits.
The objects of this invention are to provide thick film dielectric inks that hold a sufficiently high solids content to reduce the number of printing operations while maintaining lines with smooth surfaces and vias with fine definition, to provide thick film conductive and resistive inks that can continuously print fine lines with smooth surfaces, and to provide thick film inks of all types that have reduced solvent loss and a longer screen life.